1. Field of the Invention
The present invention generally relates to capacitors and particularly to a capacitor applicable to a device requiring particularly large capacitance.
2. Description of the Related Art
Attempts have been made to increase the capacitance of a capacitor having a dielectric layer interposed between a pair of electrodes. There are several techniques for increasing the capacitance of the capacitor: by increasing the dielectric constant of the dielectric material constituting the dielectric layers, by decreasing the interelectrode distance, and/or by increasing the electrode areas. One of such attempts is a capacitor including a dielectric layer composed of a dielectric material, such as a polymeric material, and conductive or ceramic fine particles having a diameter less than 1 μm dispersed in the dielectric material (refer to U.S. Pat. No. 6,544,651 and US Patent Application No. 20030006402). An example of this capacitor is shown in FIG. 6. As shown in FIG. 6, this capacitor is mainly constituted from a pair of electrodes 21 and 22 opposing each other and a dielectric layer 23 interposed between the electrodes 21 and 22. Conductive or ceramic fine particles 24 are dispersed in the dielectric layer 23. If these fine particles 24 are metal fine particles, then the thickness of the dielectric layer 23 is practically decreased, and the apparent dielectric constant determined from the observed capacitance of this capacitor becomes larger than the inherent dielectric constant of the dielectric material constituting the dielectric layer 23. When the fine particles 24 are ceramic particles, it has been reported that the dielectric constant of such a dielectric layer 23 comes between the dielectric constant of the dielectric layer 23 without any dispersed ceramic and the dielectric constant of the ceramic dispersed. Thus, the capacitance of the capacitor can be increased by dispersing fine particles in the dielectric layer 23. This apparent dielectric constant is also known as “effective dielectric constant”.
However, recent trends of faster digital circuits require capacitors to have ever larger capacitance. In the above-described capacitor, a larger amount of metal fine particles may be added to the dielectric layer to further increase the effective dielectric constant and to achieve higher capacitance. However, as shown in FIG. 7, this causes the dielectric layer to enter a so-called percolation region in the insulator-conductor transition. In this percolation region, as shown in FIG. 7, the change in dielectric constant shows a steep profile, and it is thus difficult to stably obtain a desired effective dielectric constant. Thus, a capacitor including a large amount of fine particles in the dielectric layer has a problem of low reliability. The problem of percolation does not occur when the dispersed particles are dielectric particles, such as ceramic fine particles; however, the upper limit of the dielectric constant is limited by the dielectric constant of the ceramic fine particles dispersed. This makes it difficult to fabricate a capacitor with large capacitance.